This invention relates to corpuscular radiation equipment in general, and more particularly to a magnetic lens system operating in a vacuum and comprising an improved arrangement of a shield housing, a superconducting shield, one or more lens coil windings, and a vacuum chamber for receiving the object to be examined.
One objective lens system for an electron microscope is known from U.S. Pat. No. 3,916,201. Its superconducting shielding device consists of two hollow cylindrical shielding bodies which are arranged in tandem in the beam direction axis and which closely surround the space in which the beam is conducted. The two shielding cylinders contain superconductive material which, in the operating condition, is kept below its so-called transition temperature by means of a cryogenic medium such as liquid helium. Between the adjacent end faces of these shielding cylinders a gap is developed, in which a vacuum chamber is arranged. Into this chamber an object to be examined may be introduced radially from the side by means of a separate insertion device. Because the objective space is also cooled by the cryogenic medium, lateral migration of the object due to temperature, the so-called thermal drift, can be kept extremely low, in some instances, to less than 0.03 nm/min.
Each of the two shielding cylinders is surrounded by a superconducting lens coil winding, which is short circuited in the operating condition. The effect of these shielding cylinders is that the magnetic field generated by the lens coil windings can act on the corpuscular beam only in the vicinity of the lens gap. The shielding cylinders therefore extend at their antipodal ends to regions of negligible field strength.
The two shielding cylinders are further connected to a shield housing of superconductive material which directly encloses on all sides the lens coil winding arranged around the shielding cylinders, except for the portions of the surface facing the shielding cylinders. Kept in the superconducting state, the shield housing limits the extent of the magnetic field produced by the lens coil windings, and shields the gap region to a high degree against external magnetic interference fields, particularly alternating fields.
It is well known that the resolution of corpuscular radiation equipment depends on the so-called aperture error constant of its lenses, and in particular, of its objective lens. The size of the lens gap between opposite end faces of the two shielding cylinders is therefore chosen in presently used electron microscopes so that very small values are achieved for the aperture error constant C.sub.O, the chromaticity error constant C.sub.F and the focal length f. The aperture error constant depends on the maximum value H.sub.O of the field intensity or, equivalently, the maximum value B.sub.O of the magnetic induction in the lens gap, i.e., the region in which the magnetic field acts on the corpuscular beam. The constant also depends on the field gradient along the beam direction axis in the lens gap. Assuming an approximately Gaussian axial distribution of the field intensity in the lens gap, the half-width of the Gaussian determines the field gradient for a given maximum field strength. This half-width depends on the dimensions of the two shielding cylinders used for forming the lens gap in the vicinity of their opposite end faces. Both the distance between the two shielding cylinders, i.e., the gap length in the beam direction, and the shape of the shielding cylinders in the vicinity of the opposing end faces affect the half-width.
Such an objective lens system, with an aperture error constant C.sub.O of about 1.45 mm, a gap width of 5 mm, a maximum induction of 2.1 Tesla, and a half-width of 4.4 mm, was tested in an electron microscope with a beam voltage of 220 kV. It was possible to reach the theoretical resolution limit. Cf. Optik Vol. 45 No. 3 at 291-94 (1976). The objective lens system described therein is particularly suited for electron microscopes of the so-called fixed-beam type, in which a focused electron beam, held immovable by means of magnetic fields, irradiates an object, of which a magnified image is generated by means of downstream magnetic magnification lenses.
The known objective lens system, however, is not directly applicable to the so-called transmission-type scanning electron microscopy. In this technique, a sharply focused electron beam sweeps over the surface of the object to be examined according to a predetermined raster pattern. This primary electron beam generates secondary electrons at every point of the surface. If these secondary electrons, as well as possible Auger electrons and backscatter electrons are to be collected for additional energy dispersion analsyses, then the appropriate detector devices must be arranged in the immediate vicinity of the object. However, this is not directly possible with the known objective lens system, as the object space is too small. For beam voltages under 250 kV, sufficient space can be gained by enlarging the lens gap only if an increase of the aperture error constant C.sub.O, the chromaticity error constant C.sub.F and the focal length f by about one order of magnitude can be tolerated.